rpm: set TLS priority to @LIBVIRT,SYSTEM on Fedora >= 25
rpm: set TLS priority to @LIBVIRT,SYSTEM on Fedora >= 25

With newest gnutls available in Fedora 25/rawhide, it is
possible to have TLS priority fallbacks, so we can finally
use --tls-priority=@LIBVIRT,SYSTEM

Signed-off-by: Daniel P. Berrange <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

taget/libvirt,jfehlig/libvirt,eskultety/libvirt,zippy2/libvirt,andreabolognani/libvirt,crobinso/libvirt,crobinso/libvirt,libvirt/libvirt,datto/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,jfehlig/libvirt,zippy2/libvirt,eskultety/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,taget/libvirt,fabianfreyer/libvirt,zippy2/libvirt,datto/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,taget/libvirt,fabianfreyer/libvirt,crobinso/libvirt,VenkatDatta/libvirt,libvirt/libvirt,fabianfreyer/libvirt,eskultety/libvirt,nertpinx/libvirt,eskultety/libvirt,datto/libvirt,andreabolognani/libvirt,taget/libvirt,taget/libvirt,VenkatDatta/libvirt,olafhering/libvirt,olafhering/libvirt,jardasgit/libvirt,andreabolognani/libvirt,jardasgit/libvirt,datto/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,nertpinx/libvirt,andreabolognani/libvirt,datto/libvirt,VenkatDatta/libvirt,eskultety/libvirt,jardasgit/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt